In part 3, we ll see how to interface an unamplified compensated pressure sensor with PSoC3 and evaluate the system performance. Measurement Specialties MEAS 1210 standard is used for interfacing with PSoC3. The important specifications of MEAS 1210 standard are listed below.
Supply current = 1.5 mA
Pressure Range: 0 -15 psi (Gage)
Sensitivity (max): 10 mV / psi
Sensitivity (min): 5 mV / psi
Temp error - span (max): 0.5 % FSS
Offset (max): 2 mV
Temperature error - offset: 0.5% FSS
Specified temp range: -40 to 125 °C
Bridge resistance (max): 6.4 k (50°C)
Accuracy: +/-0.1 % FSS BFSL
Sensor Operation:
The sensor is a piezo-resistive sensor excited by a current and has an output voltage proportional to the pressure and the current. The output voltage has a 50% tolerance and the sensor provides a gain set resistor to calibrate it to 1%. When the sensor is excited by proper current excitation levels specified in the datasheet, the temperature coefficient of span and offset will cause only a very small error in the final measurement (temperature compensated).
Design:
The design requires an excitation current of 1.5mA and an ADC to measure the output voltage. With a bridge resistance of 6.4k (max) and excitation current of 1.5mA (current level prescribed in the datasheet for proper temperature compensation), the load voltage of the current source is 9.6V. This means PSoC IDAC cannot directly be used for supplying bridge current because of very high load voltage. To limit external components and get maximum value out of PSoC we can use circuit below.
By controlling the VGS of this circuit, the ID can be controlled. VGS is controlled by changing the current of the sinking IDAC. RB ensures the IDAC output voltage is within compliance and optimum. The current sense resistor (0.1%), RSENS,aids in setting the current to 1.5mA. The voltage across RSENS is read by PSoC ADC (0.2%) and the IDAC current is adjusted until ID becomes 1.5mA. With this circuit, we can ensure that the current is accurate to 0.3%. A current accuracy of 2% is the requirement so the temperature error due to offset and span are within datasheet limits.
Sensor Common mode output voltage:
With this design the sensor common mode output voltage is given by;
(1.5 * 6.4)/2 + 0.150 /2 + (1.5mA * 0.05)/2 = 4.8 + 0.075 + 0.0375 = 4.91 V
Here, 1.5mA is the sensor current, 6.4k is the max bridge resistance and 0.150 V is the maximum span, 0.05 is the sense resistance.
The sensor common mode voltage is very high to directly feed into PSoC. The ADC with input buffer can accommodate only to within 200 mV of Vdda. The ADC without buffer can t be used because it has low input impedance. The PGA can allow input voltage all the way to the voltage rail, but we ll be limiting the design to supplies with very strict tolerance levels. This is not desirable as various designers might want flexibility in their power supply design (at least support 5% supplies).
Hence to lower the common mode voltage we can use a charge pump that generates a negative voltage. The generated voltage is about -3V using a negative charge pump. The ripple voltage (of <10%) on the charge pump output doesn t have a major effect as long as we set the ADC input sampling frequency as an integral multiple of the ripple frequency (the charge pump clock frequency).
ADC input range:
The sensor span is 150mV (max). The ADC input range should be > +/- 0.256V.
Resolution:
Resolution required in 1/1000th of full scale. At minimum span of 75mV, we require 75uV of voltage resolution. At 15-bit level, the ADC resolution is 64uV. With a gain of 4, the ADC resolution is < 16uV.
At +/-1.024V range, we require 15-bit resolution
At +/-0.256V range, we require 13-bit resolution
Reference:
This measurement requires an absolute reference. The final pressure accuracy depends on the reference accuracy, therefore the internal 1.024 V reference is a good choice.
The ADC has four channels:
0. Sense resistance channel: This channel is used to set the current to 1.5mA
1. Sensor Channel: Senses the sensor output
2, 3. Calibration channels: Measures the gain set resistance for calibration
The IDAC has two channels:
1. Passes current through the calibration resistance
2. Passes current through the sensor
The ADC configuration for the pressure sensing channel is shown below.
Pressure Equation:
The pressure is computed from the measured voltage using the following equation.
P = A* (Vo / Si) * Pr
P Pressure (in psi)
V0 Bridge output voltage in mV
Si Span of pressure sensor output in mV
Pr Rated Pressure (in psi)
A I/1.5. I is the actual current flowing into the pressure sensor
Calibrations required:
Span Calibration:
The Span of the pressure sensor is calibrated using the gain set resistance provided in the sensor. Using the gain set resistance, r, the span can be calibrated. The gain set resistance is trimmed such that when it s used in conjunction with a differential amplifier, it ll give a 2V span. Working the equations back, you can find that the gain set resistance.
r = (2 * Rf * Si)/ (So Si)
Here, Rf is feedback resistor of the differential amplifier, Si is the span of the pressure sensor output (differential amplifier input) and So is the span at the differential amplifier output. By looking at the datasheet of the part, Rf and So can be found. For MEAS 1210, Rf = 100k and S0 = 2V. By measuring r, we can find the span,
Si = 2/(1 + (200/r))
Performance measures:
Offset:
The sensor has a 2mV offset (max). This can be calibrated out to zero.
Span error:
The gain set resistor can provide an interchangeability accuracy of 1%. In addition, the gain set resistor can be found with 0.1% accuracy only (limited by calibration resistor accuracy. If the calibration resistor is very accurate (0.01%) or calibrated, then the span error will be 1%.
Temperature Error offset:
This has a maximum error of 0.5% FS. This is 0.075 psi.
Temperature Error span:
This has a maximum error of 0.5% FS. This is 0.075psi.
Pressure non-linearity + hysteresis:
Together they contribute 0.15% FS. This is 0.022 psi.
Signal Chain:
Offset error:
The offset error of PSoC ADC is <100uV, which can be cancelled by Correlated Double Sampling (CDS).
Offset drift:
Offset drift of PSoC is 0.55uV/°C. At 50°C, this is 11uV. It is 1/7th of minimum resolution (0.015psi). It can be cancelled by Correlated Double Sampling (CDS).
Gain error:
PSoC ADC s calibrated accuracy is 0.2%. There are 2 measurements, 1 voltage measurement and 1 current measurement (current set to 1.5mA). This can contribute to 0.4% error in total.
Gain drift:
Drift is 50 ppm/°C. For 25°C change, it ll be 0.125%.
List of all errors:
|
S.No
|
Parameter
|
Error at 10 psi (in psi)
|
|
Sensor
|
|
1
|
Offset
|
0.2 (Can be calibrated)
|
|
2
|
Span error
|
0.1 (best case)
|
|
3
|
Temperature coefficient of offset (50 °C)
|
0.075
|
|
4
|
Temperature coefficient of span (50 °C)
|
0.075
|
|
5
|
Non-linearity
|
0.022
|
|
Signal Chain
|
|
5
|
Offset
|
0
|
|
6
|
Gain error
|
0.06
|
|
7
|
Offset drift (at 50°C)
|
0
|
|
8
|
Gain drift (at 50°C)
|
0.018
|
|
9
|
INL
|
<0.015
|
PSoC Value:
Apart from integrating the analog front end, ADC, 0.1% precision reference, Op-Amp, IDAC and the MCU and providing a separate channel for accurate temperature measurement, PSoC can integrate miscellaneous features suchascapsense, segment LCD drive and communications protocols. Designing with PSoC creator reduces the design time considerably. The BOM cost and board size can also be significantly reduced.
In the next part we ll see how to interface unamplified compensated pressure sensor with PSoC3.
By Praveen Sekar